This invention relates to improved gypsum products and methods for making them. Example embodiments include methods and systems for addition of cellulose ether to a gypsum slurry that are useful to make gypsum board products such as gypsum wallboard, gypsum fiberboard, and the like (use of the term “gypsum board” herein is intended to include at least gypsum board panel, gypsum fiberboard, and gypsum wallboard products).
Gypsum board products are commonly used as building materials, with an example being wallboard and fiberboard sheets, for many reasons. They are easily manipulated to make continuous walls of desired shapes and sizes. They are durable, easily installed, patched and have beneficial insulation, fire and sound proofing properties. Decorative finishes, such as wallpaper or paint readily adhere to surfaces to allow for a large variety of decorating options.
Gypsum wallboard products are made using gypsum slurries. Water is added to calcined gypsum, also known as calcium sulfate hemihydrate, to form a pumpable and flowable slurry. The slurry is continuously deposited on a moving sheet of wallboard facing material or paperboard running at high speed, and a second sheet is then deposited over the slurry. As the slurry sets between the two sheets it converts back to its dihydrate form by absorbing and chemically reacting with water and hardens. A foam may also be added to the slurry to introduce gaseous bubbles or voids which are ultimately captured within the crystalline gypsum dihydrate. These can be beneficial to reduce the density of the gypsum board products without a significant reduction in strength.
Gypsum fiberboard sheets can be made by combining gypsum in its dihydrate form with cellulosic fibers such as wood or paper pulp. Water is added to form a slurry. The slurry is heated, typically under pressure, to calcine the gypsum and convert it to its hemihydrate form. Hemihydrate crystals form around and on the cellulosic fibers. The slurry is then deposited under atmospheric conditions on a forming wire. As water is removed on the forming wire and the slurry cools, the gypsum hemihydrate converts back to its dihydrate state and forms an interlocking solid matrix of crystals around the cellulosic fibers. In the cases of both gypsum wallboard and fiberboard, the manufacturing is typically carried out in high speed, continuous processes. Physical properties of the slurry in both cases are relatively tightly controlled in order to achieve satisfactory performance requirement of this process and the final board product.
Proposals have been made to increase the strength of gypsum board products or otherwise modify their properties. Some proposals include providing one or more additives to the gypsum slurry to affect the characteristics of the resulting dried board product. Some proposed additives, however, may affect the viscosity, tackiness or other properties of the slurry. Taking slurry viscosity as an example, excessive viscosity can lead to increased mixing time, increased energy demands, and resultant increased costs. Worse still, if viscosity increases become excessive, an incompletely mixed slurry or improperly formed board can result.
Also, addition of additives to the gypsum slurry can result in compatibility problems with other slurry components. Some additives, for example, have a surface active or surfactant functionality. These additives have the potential of destabilizing or otherwise interfering with a slurry foam additive. Some additives also have the potential of inadequate dispersion within the slurry. In the case of a viscous liquid or gel additive, for example, there is a risk that the high viscosity and limited residence time in the slurry mixer will cause an inadequate mixing.
Accordingly, problems exist in the art related to effective addition of additives to gypsum slurries.